Various types of interleaved friction plate brakes or clutches cooling means have been proposed and used with certain degrees of success.
It has been determined that a given oil flow per minute per facing square inch can sustain a certain BTU per square inch per second for a certain time before threshold damage occurs. The assumption is that gross plate area receives uniform cooling, probably achieved only in a single plate laboratory test apparatus setup. Many prior art structures disclose a series of radially extending hole passages in the plate supporting hub which act more or less haphazardly to feed cooling fluid to the plate stack. Since the plates may become located at random locations with respect to the hubs radial holes, oil does not flow uniformly across the plate surfaces, (for example, U.S. Pat. No. 3,833,100 Aschauer FIG. 1).
There is evidence that plate hot spots precede failure and may be only on a certain plate in the stack, possibly because of diminished cooling oil flow at this location by a restricted radial oil flow passage or by “path of least resistance” flow choice Uniform distribution of fluid across the plates may be achieved by excess quantities of oil; however, this is not economical and presents other problems such as released drag loss.
Most friction plate designs specify a grooved friction material surface operating against a flat smooth steel surface. Oil flow emanates outward from the inside diameter of the grooved plate. This peripheral circumference is made up of grooves and solid lands. The number of grooves, width of groove, and depth of groove defines available flow area.
This area times pressure, largely generated by centrifugal force, dictates oil flow. Pressure is usually low. Temperature rise in the oil as it passes between the mating surfaces carries away the heat of engagement.
Cooling flow for a vaporous air/oil mix is substantially different and greater than for oil for the same peripheral area (since the low density air/oil cooling medium more completely flows to all areas of the friction surface and absorbs energy by gas expansion as well as conductivity and oil particle vaporization.
The latent heat of vaporization can absorb heat energy and can be greater for vaporous air/oil than solid oil.